1. Field of the Invention
The present invention relates to group III nitride semiconductor devices (hereinbelow, also expressed simply as xe2x80x9cdevicesxe2x80x9d), and more particularly to a method of fabricating a semiconductor laser devices which employs a group III nitride material system.
2. Description of the Related Art
A laser device requires a pair of reflectors or reflecting mirrors for forming an optical resonator to operate. In the case of fabricating semiconductor laser devices (of Fabry-Perot type) using the semiconductor materials such as GaAs etc., the reflecting mirrors are mostly formed by the cleavage of GaAs crystal substrates.
The crystal system of group III nitride semiconductors is one similar to a hexagonal system, called xe2x80x9cwurtzite typexe2x80x9d, unlike the sphalerite type of group III-V semiconductors, but it also has a definite cleavage plane. It is accordingly the best to form a laser device structure on, for example, the GaN bulk crystal substrate.
However, in the case of fabricating a semiconductor laser device by the use of the group III nitride materials, a nitride bulk crystal to be employed as the substrate has not been produced yet. Therefore, the device is inevitably fabricated by expitaxially growing a nitride crystal film as an underlayer on a different kind of substrate of sapphire, SiC or the like.
Heretofore, as methods of fabricating the reflector surfaces of nitride lasers on substrates, the following four 1)-4) have been known:
1) A laser structure of grown nitride films is fabricated on a substrate, and it is shaved by dry etching such as reactive ion etching (RIE), thereby to obtain reflector surfaces. Fabricating a laser structure by growing nitride films on a substrate, then forming reflector mirrors by dry etching such as reactive ion etching (RIE).
2) Growing nitride films the C-plane, namely, (0001) plane or the A-plane, namely, (11{overscore (20)}) plane (hereafter referred to as (11-20) plane)of a sapphire substrate, and splitting the wafer along the (1{overscore (100)}) plane (hereafter referred to as (1-100) plane) or (1{overscore (102)}) plane (hereafter referred to as (1-102) plane) of sapphire substrate, thereby obtain reflector mirrors.
3) Growing a laser structure on a SiC substrate, and thinning the back surface thereof, and cleaving the resultant structure along with the substrate, thereby obtaining reflector mirrors.
4) After growing a thick, for example 100 xcexcm-thick GaN film on a sapphire substrate, removing the sapphire substrate by grinding or lapping, then using the remaining GaN film a substrate crystal on which a laser structure is formed.
Favorable single-crystal films have ever been obtained on the C-plane and A-plane of sapphire. The sapphire substrate is very difficult to be split as compared with a GaAs substrate etc. which have hitherto been employed for a semiconductor laser etc. It has therefore been common practice to avoid the method based on the cleavage, and to obtain the reflective surfaces by the etching (RIE). The sapphire does not have a clear cleavage plane like those of Si, GaAs etc. Regarding the C-plane, however, the sapphire can be tentatively split along the (1-100) plane. Also, regarding the A-plane, it can be split along the (1-102) plane, namely, a so-called xe2x80x9cR-planexe2x80x9d favorably in a state considerably close to the cleavage of the ordinary crystal.
Nevertheless, the respective methods 1)-4) have disadvantages as stated below.
Regarding the forming method 1) which employs the RIE, it is difficult to obtain reflective surfaces perpendicular to the waveguide, and also hard to obtain smooth surfaces favorable for the reflector mirrors. Accordingly, the method 1) has the problem that the far field image of emitted light forms multiple spots. In particular, the formation of the multiple spots of the emitted light is ascribable to the fact that the sapphire cannot be effectively etched even by the dry etching such as RIE. As shown in FIG. 1, in a laser device with the reflector surfaces 2 formed by etching, portion of the emitted beam is reflected by the part of a sapphire substrate 3 indicated by (S) in the figure (the part left without being etched), and the reflected light interferes with the main beam, so that the far field image forms the multiple spots. The formation of the multiple spots in the far field image is fatal to a light source for an optical disk system, and hence, a laser device thus fabricated cannot be put into practical use at all.
In the case of forming method 2), the growth on the sapphire C-plane has the troublesomeness that the structure cannot be split unless the sapphire substrate is thinned by polishing the back surface thereof, and has the problem of low reproducibility in the splitting process. These problems are ascribable to the fact that the sapphire (1-100) plane is not the genuine cleavage plane. Since the sapphire is a very hard crystal, it cannot be split along scribing lines without being thinned. More specifically, when it is intended to obtain split surfaces which are practical for a laser device, the sapphire substrate needs to be thinned down to about 100 xcexcm. In the case of polishing the back surface of the substrate on which a laser structure has already been formed the wafer is warped and distorted by the difference between the thermal expansion coefficients of the sapphire and nitrides, or by a residual stress attendant upon the polishing. On account of the warp and the distortion, wafer breakage is very prone to occur during the polishing process. This is very disadvantageous for mass production.
The crystal orientation of the GaN grown on the sapphire C-plane rotates by 30 degrees relative to that of the substrate. Accordingly, when the sapphire substrate is split along the (1-100) plane, the overlying GaN is to be split along the (11-20) plane. Since the cleavage plane of a GaN crystal is the (1-100) plane, the GaN is somewhat forced to be split along the crystal plane not being the cleavage plane, in this case. Owing to the symmetry of the GaN crystal, however, a very good fissured surface is obtained when the splitting is in a direction precisely along the (11-20) plane.
Meanwhile, since the (1-100) plane is not the cleavage plane, the sapphire can also be split even when a scribing line is drawn with a deviation. In this case, the GaN is to be split in a direction deviating from the (11-20) plane.
Therefore, low reflectivity and irregularity in the wave front of emitted light are incurred to deteriorate the quality of the mirror facet for a laser.
Further, the growth on the sapphire A-plane in the forming method 2) has the problem that the quality of the fissured surface of the GaN is unsatisfactory.
Since the R-plane being the (1-102) plane which is the parting plane of the sapphire, the A-plane sapphire can be easily parted even with a thickness of 250-350 xcexcm ordinarily applied to a substrate. However, in the case where, as shown in FIG. 2, a laser structure is formed on the A-plane of the sapphire substrate and is parted from the direction indicated by an arrow, a plurality of fine striations appear on the side surface of the GaN. The appearance of the plurality of striations is ascribable to the fact that the sapphire substrate constitutes most of the thickness of the wafer and the cracks therefore propagates along the R-plane of sapphire crystal. Although the sapphire substrate cracks along its R-plane, the (1-100) plane of the GaN grown on the sapphire A-plane deviates by 2.4 degrees from the sapphire R-plane. Therefore, even after the crack has reached a sapphire/GaN interface, it propagates into the GaN crystal along the R-plane of the underlying sapphire to a slight. However, the GaN tends to split along the (1-100) plane being its cleavage plane, such a plurality of (1-100) planes form a stepped fissured surface. Therefore, the second method of fabricating the reflector surfaces in accordance with the growth on the sapphire A-plane is also disadvantageous in that the quality of the fissured surface of the GaN does not become very good.
With the method 3), the SiC substrate is very expensive, and this leads to the problem of a heavy burden in making various studies on growth conditions etc. Besides, in polishing the back surface before the cleaving step, conspicuous difficulty is involved because the hardness of SiC is very high. Further, the nitride layers which are formed on the SiC substrate are prone to crack in relation to the difference of thermal expansion coefficients, and this leads to a limitation on the design of the thickness of the nitride layers.
The method 4) is ideal concerning the cleaved state as explained before. However, it is difficult to form the thick GaN layer by vapor growth, and the step of polishing away the sapphire is very troublesome. Therefore, it has not been attained yet to obtain crystal substrates of large diameter with a sufficient yield.
Therefore, the object of the present invention is to provide a reproducible method of manufacturing a group III nitride semiconductor laser having high quality reflector surfaces.
A fabrication method according to the present invention is a method for producing a nitride semiconductor laser which is obtained by successively stacking a plurality of crystal layers made of group III nitride semiconductors, including an active layer, on an underlayer made of a group III nitride semiconductor, is characterized by comprising:
the crystal layer formation step of forming the plurality of crystal layers on the underlayer formed on a substrate;
the electrode layer formation step of forming an electrode layer on an outermost surface of said crystal layers;
the plating step of plating a metal film onto the electrode layer;
the light irradiation step of irradiating an interface between the substrate and said underlayer with light through the substrate to form a region of decomposed substance of the nitride semiconductor;
the delaminating step of detaching said underlayer that supports said crystal layers from said substrates along the decomposed substance region; and
the cleavage step of cleaving said underlayer with said crystal layers thereon to form cleaved mirrors of laser resonator.
In an aspect of the fabrication method according to the invention, said plating step includes a step of forming insulating stripes which extend parallel to an extending direction of the cleavage planes to be formed in said nitride semiconductors, on said electrode layer before the plating.
In another aspect of the fabrication method according to the invention, the scribing lines are formed on crystal layers parallel to the insulating stripes, whereupon said underlayer with said crystal layers thereon is cleaved at said cleavage step.
As to a further aspect of the fabrication method according to the invention, the metal film is made of copper.
As to a still further aspect of the fabrication method according to the invention, in said light irradiation step, light to be used (applied) is selected from a group of light having a wavelength which passes through said substrate and which is absorbed by a part of said underlayer vicinal to the interface.
In another aspect of the fabrication method according to the invention, at said light irradiation step, the irradiation is performed uniformly over the interface between the substrate and the underlayer, or by scanning the interface with a spot or with a line of light.
In a further aspect of the fabrication method according to the invention, said crystal layer formation step includes the step of forming waveguides which extend perpendicular to the cleaved planes to be formed in said nitride semiconductors.
As to a still further aspect of the fabrication method according to the invention, said crystal layers are formed by metal organic chemical vapor deposition.
As to another aspect of the fabrication method according to the invention, in the light irradiation step the light beam toward the interface, the light beam applied is an ultraviolet ray generated from a frequency quadrupled YAG laser.
A nitride semiconductor laser according to the invention having a plurality of crystal layers made of group III nitride semiconductors including an active layer comprises:
an underlayer made of a group III nitride semiconductor on which the crystal layers are successively stacked; and
a plating metal film plated on an opposite side to the underlayer with respect to said crystal layers.
As to one aspect of the nitride semiconductor laser according to the invention, the plated metal film has suture planes each substantially coinciding with a cleavage plane of the stacked crystal layers of nitride semiconductors to constitute a laser resonator.
As to another aspect of the invention, the nitride semiconductor laser further comprises a waveguide which extends perpendicular to the cleavage plane.
As to a further aspect of the nitride semiconductor laser according to the invention, the plated metal film is made of copper.
As to a still further aspect of the invention, the nitride semiconductor laser further comprises a heat sink onto which a side of the underlayer is bonded.
As to another aspect of the invention, the nitride semiconductor laser further comprises a heat sink onto which a side of plated metal film is bonded.
A method for separating a substrate from a nitride semiconductor wafer, according to the invention, which is obtained by successively stacking at least one crystal layer made of group III nitride semiconductor on the substrate, comprises the steps of:
forming an auxiliary substrate on an outermost surface of the crystal layer;
irradiating an interface between the substrate and the crystal layer with light through the substrate to form a region of decomposed substances of the nitride semiconductor; and
detaching said crystal layer away from said substrate along the decomposed substance region.
As to one aspect of the separation method according to the invention, the step of forming an auxiliary substrate includes a step of plating a metal film as the auxiliary substrate onto the outermost surface of the crystal.
As to another aspect of the separation method according to the invention, the step of plating includes a step of forming an electrode layer on the outermost surface of said crystal layer before plating;.
As to a further aspect of the separation method according to the invention, the metal film is made of copper.
As to a still further aspect of the separation method according to the invention, in said light irradiation step, light to be used(applied) is selected from a group of light having a wavelength which passes through said substrate and which is absorbed by a part of said crystal layer vicinal to the interface.
As to another aspect of the separation method according to the invention, at said light irradiation step, the irradiation is performed uniformly over the interface between the substrate and the crystal layer, or by scanning the interface with a spot or with a line of light.
As to a further aspect of the separation method according to the invention, in said light irradiation step, light to be applied is an ultraviolet ray generated from a frequency quadrupled YAG laser.
According to the present invention, the decomposition region in which the crystal coupling between the sapphire substrate and the GaN crystal is entirely or locally released is formed, whereby the sapphire substrate can be removed away from the GaN crystal underlayer while holding the fabricated structure by the GaN crystal side, so that the nitride semiconductor laser can be reliably obtained.
Since, at the cleavage step, a crystal part to be parted is substantially made of only GaN-based materials, a cleaving property can be improved to obtain a stable reflector surface of good quality. In order to keep the strength of the GaN-based crystal portion, the metal film which acts as an auxiliary substrate is formed on the laser structure. The auxiliary substrate is formed by a plating method in which the electrode of the laser structure under fabrication is employed as an underlying electrode and is overlaid with a metal material by electrodeposition. The plating method includes elecroless plating in addition to electroplating.
Further, at the plating step, insulating stripes each of which is a long and narrow pattern made of an insulating material are formed on the parts of the underlying electrode corresponding to lines along which the GaN-based crystals are to be cleaved. Owing to the formation of the insulating stripes, the strength of the cleavage line parts of the auxiliary substrate can be locally lowered along the cleavage line.
After the deposition of the metal film, namely, the formation of the auxiliary substrate, the output of a high-power ultraviolet laser is applied from the back side of the sapphire substrate, thereby separating the sapphire substrate from the GaN-based crystal layers. An electrode is formed on a surface exposed by the removal of the sapphire substrate. Thereafter, the metal film is divided along the above insulating stripes, and the GaN-based crystals are simultaneously cleaved, whereby laser bars are obtained.